1. Field of the Invention
The present invention relates to an image reading apparatus and, more specifically, to an image reading apparatus capable of eliminating moire pattern generated in reading dot images.
2. Description of the Prior Art
A digital copying machine has been known in which an original image is read by an image pickup device and the image is printed on a paper by a laser printer and the like based on the image data obtained therefrom.
However, there arises a problem in reading an original image such as a photograph or a painting having thick portions and thin portions provided as a dot image by a half tone etching method. More specifically, because of the relation between the pitch between each of the dots and the reading pitch by the image pickup device, or of a subtle deviation of phase based on the period and the like in half tone processing, a periodical pattern of thick and thin portions called moire pattern is generated, providing trouble in viewing.
A method has been proposed to prevent the generation of the moire pattern, in which dimension or pattern of a Dither matrix is changed in half tone processing. However, the moire pattern cannot be eliminated by this method when the reading pitch (the pitch between pixels in the image pickup device) itself is the cause of the moire pattern.
Under another method, the moire pattern has been eliminated by arranging a filter for eliminating the moire pattern in a light path in reading the original image and by gradation of the image by dispersing the image focused on one pixel of the image pickup device onto adjacent pixels. However, the moire pattern cannot be eliminated in all of the images having thick portions and thin portions even by this method when a plurality of images with thick portions and thin portions having different dot pitches are included in the original image.
In view of the foregoing, the applicant of the present invention proposed in the above mentioned related application, an image processing apparatus capable of eliminating moire pattern employing optical enlarging means for enlarging an original image to be read by an image pickup device and electrical reducing means for reducing the image by thinning out image data read by the image pickup device. Accordingly, by adjusting the rates of magnification of the optical enlarging means and the electrical reducing means in association with the dot pitch of the original, the moire pattern can be effectively eliminated. However, in the image processing apparatus of the prior application, the magnification rate to be set for the optical enlarging means and the electrical reducing means must be determined by trial and error process every time the apparatus is used. Such adjustment requires a number of setting of the magnification rate and a number of trial prints in order to eliminate the moire pattern. Much time and labor is required for effective elimination of the moire pattern.
For a user employing dot screens for providing pictures and photographs, and where such user knows the number of lines (per inch) of the screen used, it is very convenient if the magnification rate for eliminating the moire pattern can be set by using the number of lines of the screen.